1. Field of the Invention
The present invention relates to a process for producing a flexible polyurethane foam. More particularly, it relates to a flame-retardant weldable polyurethane foam having a skin improved in stain resistance.
2. Description of the Prior Art
Polyurethane foam is in general use as cushioning materials for vehicles, furniture, bedding and the like because of its superior elastic properties. It also finds use as building materials and heat insulation materials. Polyurethane foam used as cushioning materials is usually covered with a skin material such as cloth or leather which is welded to the surface thereof.
Since polyurethane foam is rather combustible, studies have long been made to render it flame retardant. One way of achieving this object is to incorporate a proper flame retardant into the raw materials of polyurethane foam. Flame retardants in common use are as follows:
(a) Liquid flame retardants based on halogen-containing phosphate ester or phosphite ester; e.g., tris-2,3-dichloropropyl phosphate.
(b) Powder flame retardants composed of antimony trioxide and halogen-containing compound; e.g., a composition of antimony trioxide and vinyl chloride.
(c) Phosphorus-containing compounds or halogen-containing compounds used alone; e.g., chlorinated paraffin.
The flame retardant belonging to the above category (a) is good in flame retardance and has only a small effect on the welding of skin materials; however, it has a disadvantage of staining the skin material. In other words, it migrates to the skin material at the time of welding and stains it gradually with time. The flame retardant belonging to the above category (b) is good in flame retardance and has only a slight tendency toward skin staining; however, it impairs the weldability completely. The flame retardant belonging to the above category (c) is less effective than others in flame retardance and is required to be added in large amounts to polyurethane foam. Moreover, it has a disadvantage of markedly impairing the weldability.
With the conventional flame retardants, it was difficult to improve the flame retardance of polyurethane foam while keeping the weldability and avoiding the skin staining.